1. Field of the Invention
The present invention relates to a magnetic disk drive and to a head position control method for a magnetic disk drive, and more particularly to a magnetic disk drive and to a head position control method for a magnetic disk drive which are capable of performing high-speed seek and high-accuracy positioning, even if a servo track which is defined by servo information recorded on the surface of the disk is eccentric with respect to the center of rotation.
2. Description of Related Art
In recent years, to achieve an increase in the storage capacity of magnetic disk drives, efforts have been made to achieve even higher density, with an accompanying need to improve access speed. Various methods have been proposed to achieve an increase in magnetic disk drive storage capacity and an improvement in access speed.
In a magnetic disk apparatus (hereinafter referred to simply as a disk drive), a magnetic head (hereinafter referred to simply as a head) records data onto a magnetic disk (hereinafter referred to simply as a disk) by recording a magnetic pattern, and reads out data that has been recorded onto the disk by detecting magnetic field variations occurring due to this recorded magnetic pattern. To establish the position for recording and reading out, a magnetic guide on the disk known as a track is recorded onto the disk coaxially with respect to the center of rotation of the disk. By specifying a track, the position in the radial direction is identified. Each track on the disk is divided in the circumferential direction into a plurality of parts known as sectors, the sector numbers of which identify each of the sectors being recorded onto the disk. Recording of data is performed in units of sectors and, by specifying a sector number, it is possible to identify the circumferential position. In a disk drive which has a plurality of disks having the same rotational axis and a head provided for each recording surface of the disk, the heads are linked so as to move in concert, one of the disk surfaces, for example, being made a servo surface, onto which information related to tracks and sectors is recorded, with only data being recorded onto the other disk surfaces, in what is known as a dedicated servo system. However, in this dedicated servo system, the head position with respect to the servo surface has a head position error with respect to the written data position. Because of variations in temperature, vibration of the head movement mechanism, and the like, it is not possible to make this error very small. Therefore, it was difficult to increase the density by making the track spacing sufficiently narrow. For this reason, in recent years data has been written on all disk surfaces, with information related to the track recorded at the very beginning of the sectors of each disk surface, this being detected so as to control the head position. This system, in which servo information is recorded together with data is known as an embedded servo system.
In current disk drives, a disk is mounted on the rotating shaft of a spindle motor, servo information is recorded by means of a servo track writer (STW) so as to perform track writing initialization. The writing and readout of data are performed after performing this initialization operation. In reality, an STW program is started and servo information is written onto the disk surface by a head. Thus, the path of a track is a circle which has as its center the center of rotation. This applies to both the embedded servo system and the dedicated servo system.
However, in reality, because the head and disk are vibrating while the track is being written, the track is not a true circle, but rather vibrates about a circular path. In order to improve the recording density of a disk apparatus, it is necessary to increase the number of tracks that can be recorded on a disk. However, when the disk vibrates, because of the risk of mutual interference between adjacent tracks, it is not possible make the track spacing very narrow, thereby presenting an obstacle to the improvement of recording density.